High power semiconductor laser diodes

ABSTRACT

A high power laser source comprises a bar of laser diodes having a first coefficient of thermal expansion CTE bar  on a submount having a second coefficient CTE sub  and a cooler having a third coefficient CTE cool . The submount/cooler assembly shows an effective fourth coefficient CTE eff  differing from CTE bar . This difference leads to a deformation of the crystal lattice of the lasers&#39; active regions by mechanical stress. CTE eff  is selected to be either lower than both CTE bar  and CTE cool  or is selected to be between CTE bar  and CTE cool . The submount may either comprise layers of materials having different CTEs, e.g., a Cu layer of 10-40 μm thickness and a Mo layer of 100-400 μm thickness, or a single material with a varying CTE sub . Both result in a CTE sub  varying across the submount&#39;s thickness.

This application is a divisional of U.S. patent application Ser. No.12/873,382, filed Sep. 1, 2010, which issued on Nov. 27, 2012, as U.S.Pat. No. 8,320,419, which is a continuation in part of U.S. patentapplication Ser. No. 12/233,658, filed on Sep. 19, 2008, now abandonedwhich claims the benefit of U.S. Provisional Patent Application No.60/973,936, entitled “High Power Semiconductor Laser Diodes,” filed Sep.20, 2007.

FIELD OF THE INVENTION

The present invention relates to semiconductor high power laser diodedevices, in particular to the cooling system of broad-areasingle-emitter (BASE) laser diode devices. They include laser diodesarranged in a bar structure of up to 30 and more diodes and are nowcommonly used in many industrial applications. Such a laser device mayproduce 100 W or more of light power, each of the laser diodes producingat least 10 mW output. At powers of this magnitude, it is important tomanage heat dissipation in order to achieve good product performance andlifetime. Usually, such a laser diode bar is arranged on a submount,mostly junction side down, which submount transfers the heat of thelaser bar to a cooling system. Mechanical stress, introduced to thelaser diode bar as a result of the assembly on a cooling platform, has asignificant influence on fundamental device properties such asreliability, polarization purity, spectral width, and curvature of thehorizontal light emission line, so-called “smile”. The present inventionaddresses the adjustment of the assembly to the laser bar properties bysubmount design.

BACKGROUND AND PRIOR ART

Today, one major problem when manufacturing industrial laser bars is thelarge mismatch in thermal expansion coefficient (CTE) between thecommonly used laser diodes and the cooler. For example, GaAs-based laserdiode bars have a CTE=6.5×10⁻⁶ K⁻¹, whereas the usual copper cooler hasa CTE=16×10⁻⁶ K⁻¹.

There are three common mounting technologies for industrial laser barson copper coolers:

(1) The laser bar is directly attached to the copper cooler using a“soft solder”, e.g. In, InAg, or InSn.

(2) The laser bar is attached to a “CTE-matched” CuW submount,consisting e.g. of a homogenized 10% Cu and 90% W submount, forming abar-on-submount structure (BoS), using a “hard solder”, e.g. AuSn, andthen

-   -   (2a) mounting the BoS on the copper cooler using a “soft        solder”, e.g. In, InAg, or InSn, or    -   (2b) mounting the BoS on the cooper cooler using a “hard        solder”, e.g. AuSn, SnAgCu, or PbSn.

For the following reasons, none of these three mounting technologiesresults in a satisfactory assembly for industrial laser bars:

One reason is the insufficient stability of the solder interface whichresults in an unsatisfactory reliability. A drawback of “soft” (i.e. lowmelting point) solders is their instability under thermal cyclingoperation, e.g. on-off operation common in industrial laserapplications. As a consequence, with the mounting technologies describedin (1) and (2a) above, the limiting operating condition is notdetermined by the properties of the laser diodes, but by the poorstability of the solder interfaces. Tests have shown that for oneparticular diode design, the maximum drive current for a reliableoperation is about 90 A when using the mounting technology (1), i.e.direct mounting the diode onto the copper cooler using In. For thetechnology (2a), the maximum drive current is 120 A, i.e. mounting theBoS on the copper cooler using InAg. When using hard solder only asdescribed in (2b), it is 180 A. As a consequence, “soft solder”technologies seem to be no option for future industrial laser bargenerations to meet the market requirement of a very high optical outputpower.

For the temperature-induced deformation of a laser bar caused by thetemperature difference between the mounting condition and the usecondition on or with its mount or submount, persons skilled in the artuse the term “smile” as a descriptor because of its appearance. “Smile”of a laser device in this context is defined as the warping or curvatureor bow of a laser device along the length of the laser diode bar whichis in the plane orthogonal to the emitted light beam, i.e. orthogonal tothe emitted light beam. Thus, looking head-on into the light-emittingfacets of the laser diodes of the bar, the various facets do not form astraight line. Smile is generally believed to result from stress and theterm is often used to imply that the device has been subject to thermalstress.

Because technology (1) avoids a submount, it allows the design ofdevices with better thermal conductivity than comparable devices usingthe technologies (2a) and (2b). Also, because of the low soldertemperature and the ductility of the soft solder, devices assembledusing this technology have low bow values, i.e. <2 μm. Further,vertically stacked laser bar arrays for very high power output may bemade smaller, thus enabling better and easier vertical collimation ofthe laser beam by lenses or other optical means.

However, as mentioned above, the limited reliability of soft-soldereddevices in off-on operation is an important drawback of this technology.

Technology (2a) uses a submount which is CTE-matched to the laser barand a ductile soft solder between the various parts. This results inlow-bow and low-stress devices. Further, such devices are significantlymore reliable than comparable devices assembled with technology (1).This behavior is based on the fact that, because of the missing submountin technology (1), the soft In-based solder interface is close to thelight/heat-generating region responsible for thermal andthermo-mechanical driving forces, which, for an on-off operation mode,cause a degradation of soft-solder interfaces. These driving forces aredirectly correlated to the spatio-temporal temperature distribution inthe solder interface. Because of the thermal spreading within thesubmount, the temperature distribution is more homogeneous fortechnology (2a) than for technology (1), where there is no submountacting as a heat spreader between the heat-generating region and thesoft solder interface. Nevertheless, the maximum reliable operationpower of devices assembled using technology (2a) is in many casesdetermined by the stability of the soft solder interface. “Hard solder”assembly technologies avoid this problem and achieve more reliableoperation of high power devices.

Technology (2b) offers such a pure hard solder assembly. The CuWsubmount, having a thermal expansion coefficient (CTE_(sub)) equal orclose to the thermal expansion coefficient (CTE_(bar)) of the laser bar,acts as a stress buffer between the copper cooler and the laser bar.Nevertheless, the limitation of the thermal expansion coefficient to anarrow region centered at 6.5×10⁻⁶K⁻¹ often leads to non-optimizeddevice characteristics, such as high smile values, undefined spectralshape or poor polarization purity.

Further, stress within and the often resulting smile of a laser devicehave a significant impact on the reliability. For some devices, e.g.devices having a stress-sensitive epitaxial structure, technology (2b)often leads to reliability problems, because e.g. a hard solder and aCuW submount are unable to compensate the uncontrolled compressivestress in the device caused by the thermal mismatch between thelaser/submount and the cooler.

Also, to control the CTE-mismatch between diode and cooler, so-calledCTE-matched coolers have been developed. Known technologies forCTE-matched coolers are:

-   -   CuMoCu micro channel coolers;    -   Cu—AlN micro channel coolers; and    -   Al—C (nanotubes) passive coolers.

Although these coolers are technically quite advanced, they have somedisadvantages:

-   -   they are expensive and are therefore used only for demonstration        or “niche” applications;    -   some users expect cooler reliability problems and therefore        hesitate to switch to a CTE-matched cooler; and/or    -   the thermal conductivity of the CTE-matched coolers is in        general not as good as the thermal conductivity of a copper        cooler with the same geometry.

Also, layered submounts have been developed to obtain a better matchbetween the laser diode bar and the cooler, i.e. these layered submountsaim to match the CTE_(bar) of the laser bar to reduce the stress to thelatter. This may seem to solve the problem but it does not. The reasonis that the application of heat when soldering the laser bar to thelayered submount inevitably introduces uncontrolled stress.

A specific example of a high power laser mounting module discloses Hadenin U.S. Pat. No. 5,848,083. This module includes a bulk layer withstress-relief apertures and consists of at least two components: amounting plate and a multi-layer heat transfer component. The cooler ismounted to the mounting plate. Apart from being rather complex and thuscostly, the main object of Haden's design is to produce an“expansion-matched, stress-relieved” module with high thermalconductivity. The CTE of this design is said to be “substantially equal”to the CTE of the “heat dissipating element”, i.e. the laser bar. Inother words, Haden discloses an approach equivalent to the type (2b)technology described above.

However, all prior art solutions are focused on the avoidance orminimization of stress in the laser bar and address potential solutionstherefore. To summarize, despite the various partial solutions for thestress problem of laser diode bar devices, there is still a need for asimple, cost-effective design of such devices.

THE INVENTION

It seems that one specific point is overlooked by the solutionsdisclosed in the prior art. The following invention overcomes theproblems remaining when using the prior art.

As mentioned above, the prior art solutions and approaches aim atreducing or avoiding mechanical stress to the laser bar. They do noteven consider that a controlled stress exerted to the laser bar in thefinal device may improve the quality of the optical output and/or thereliability. In other words, stress in the assembled laser device may bean advantage—not a problem. The present invention focuses on definingmechanical properties of the substructure, essentially of the submountand the cooler, such that the stress in the assembled laser device isnot minimized, but is adjusted to the requirements and properties of thelaser device.

This approach generates a greater degree of freedom for the design oflaser devices because the laser assembly solutions described in theprior art pre-define the compatibility between laser properties andassembly stress and therefore limit the choice of parameters for e.g.epitaxial design, metallization, or dielectric layers.

For describing the stress in the light emitting region of a laser bar, aparameter called CTE_(eff) is introduced. This parameter describes themechanical impact of submount and cooler, i.e. the submount coolerassembly, on the assembled bar. Based on Finite Element Modelinginvestigations, a linear relation between CTE_(eff) and the engineeringstrain Δa/a of the light emitting semiconductor region was established.A typical value is Δa/a=0.0001 (i.e. 0.01% assembly strain) for aCTE_(eff) deviating by 5% from CTE_(bar). X-ray diffraction orspectroscopic methods such as photoluminescence or photocurrentspectroscopy are suitable to quantify the engineering strain in thelight emitting region with the required resolution and sensitivity asdescribed by Jens W. Tomm, Tran Quoc Tien, and Daniel T. Cassidy inAPPLIED PHYSICS LETTERS 88, 133504, 2006.

To achieve such a “strained laser bar device”, a submount/coolerassembly is produced whose CTE_(eff) differs from the CTE_(bar) in aparticular way as will be explained below. This is just the opposite ofmost mainstream approaches, which consist in producing a substructure,e.g. a submount/cooler assembly, whose CTE is as close as possible orequal to the CTE_(bar) or which use a ductile solder to compensate forthe CTE mismatch between the laser bar and the substructure.

In brief, the invention is a laser source with the various coefficientsof thermal expansion CTE_(bar), CTE_(sub), CTE_(cool), and CTE_(eff) asdefined above whose laser bar is intentionally stressed. More precisely,its laser bar's semiconductor crystal lattice is deformed by mechanicalstress executed upon said laser bar by the submount/cooler assembly.

This stress is preferably produced by choosing a submount/coolerassembly with a CTE_(eff) different from the laser bar's CTE_(bar),CTE_(eff)≠CTE_(bar). It was found that a minimum CTE difference of 5%effects a deformation of the crystal lattice of about 0.01%. However,the CTE_(eff) may differ much more from CTE_(bar), e.g. up to 100%.

Thereby, CTE_(sub) may be selected higher than said first coefficientCTE_(bar) and smaller than or equal to said third coefficient(CTE_(cool)), in short: CTE_(sub)>CTE_(bar) and CTE_(sub)≦CTE_(cool).

Alternatively CTE_(sub) may be selected less than both CTE_(bar) andCTE_(cool), in short: CTE_(sub)<CTE_(bar) and CTE_(sub)<CTE_(cool).

A method for making a high power laser source according to the inventionincludes choosing CTE_(eff) different by a predetermined amount fromCTE_(bar), CTE_(eff)≠CTE_(bar), hard soldering the bar of laser diodesto the submount and also hard soldering said submount to said coolingelement. Thereby the laser bar's active region is stressed whichproduces a deformation of its semiconductor crystal lattice.

More features of the invention are disclosed in the followingdescription of details and embodiments and in the claims.

This means that the present invention uses an approach totally differentfrom the prior art in that it intentionally exerts stress to the laserbar and, by positively controlling and/or adjusting this stress, therebyimproves the laser device.

This may be effected by selecting the CTE_(sub) of the submount and theCTE_(cool) of the cooling element such that the CTE_(eff) of thesubmount/cooler assembly deviates from the CTE_(bar) of the laser bar bya certain amount or percentage. More precisely, the CTE_(eff) of thesubmount/cooler assembly can be selected either larger or smaller thanthe CTE_(bar) of the laser bar. By choosing the suitable differencebetween CTE_(bar) and CTE_(eff), either positive or negative, a“stress-adjusted” laser device with optimal properties is achieved. Itshould be noted that the stress exerted on the laser bar may be eithercompressive or tensile. The following examples explain this inventiveconcept and present general rules for implementing the invention. Moredetailed embodiments are described together with the drawings.

With a given CTE_(bar) of the laser bar, there are two ways forselecting the CTE_(sub) of the submount and the CTE_(cool) of thecooling element:

-   -   either selecting CTE_(sub)>CTE_(bar) and CTE_(sub)≦CTE_(cool),    -   or selecting CTE_(sub)<CTE_(bar) and CTE_(sub)<CTE_(cool). In        both cases, the resulting CTE_(eff) of the submount/cooler        assembly must differ from the CTE_(bar) by at least 5%. The        upper limit may be 100%, i.e. for a particular design an optimal        CTE_(eff) of the submount/cooler assembly may be twice or half        the CTE_(bar).

Selecting the various CTE values according to these basic inventiverules results in a stress-adjusted high power laser device. The examplesdescribed below illustrate details of such optimal designs.

Typically, a high power diode bar has a CTE_(bar) much lower than thatof the cooler. As mentioned above, a GaAs diode bar has aCTE_(bar)=6.5×10⁻⁶K⁻¹ compared with a usual copper cooler withCTE_(cool)=16×10⁻⁶K⁻¹.

The invention requires, of course, a selection of materials andthicknesses of the components used. Since the material of the laserdiode bar is usually selected according to the desired output (power andfrequency) and the material of the cooler is often given by designrestriction or customer requirement, only the material of the submountcan be selected according to its thermal and mechanical properties.

As explained above, according to this invention, the submount, its CTEand/or structure is tailored in such a way that, in the final assembly,the submount exhibits a predetermined force on the mounted laser diodebar, i.e. the laser bar is stressed or preloaded in the final device.

A particular feature is to design the submount as a layered structure,e.g. as CuMoCu structure. Although layered submounts are not new per se,they have not been pre-stressed or preloaded according to the inventionuntil now.

A further feature is to design such a layered submount advantageouslyasymmetrically, e.g. as a MoCu layer with the Cu layer facing the cooleror as a CuMoCu sandwich with two Cu layers of differing thicknesses, thethicker Cu layer facing the cooler.

Another particular feature of this invention is to provide alaser/submount subunit which is pre-stressed, e.g. already bent, i.e.shows a smile. This may be done by bending the submount beforesoldering, e.g. by an asymmetric design of the submount, in which casethe submount consists of a vertically asymmetric arrangement of layerswith different CTEs. Pre-bending may also be accomplished by mechanicalmeans before or during assembly, e.g. by optimizing the curvature ofsolder tools (thermode/base plate, stamps).

As a result of this new approach of submount design is that it allowsthe mounting of laser diode bars using hard solder technologies on anysubmounts, especially of InGaAlAs-based laser bars on copper or othercoolers with similar CTEs. This results in the following benefits andadvantages:

-   -   defined stress in the active region of the lasers which results        in high reliability of the laser device, precise spectral width,        and high polarization purity;    -   low smile values, i.e. minimum deformation, which results in        better beam shaping and better coupling to the usual wave guide;    -   stable solder interfaces which again improves the reliability of        the laser device.

This altogether provide much greater freedom for the design of laserdevices than prior art approaches and allows to increase the ratedoutput power of laser devices without introducing smile or poorreliabilty or undesired optical behavior.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, embodiments of the invention will be described byreference to the drawings, which show in:

FIG. 1 a a schematic drawing of a laser bar/cooler structure with a softsolder, e.g. prior art technology (1);

FIG. 1 b a schematic drawing of laser bar mounted on a CTE-matchedsubmount using hard solder which submount is affixed to a copper coolerusing a soft solder, e.g. technology (2a);

FIG. 1 c a schematic drawing of laser bar mounted on a CTE-matched CuWsubmount and a copper cooler using hard solder on both interfaces, e.g.technology (2b);

FIG. 1 d a schematic drawing of laser bar mounted on a “stress-tailoredsubmount” according to the present invention, which submount is affixedto a copper cooler using hard solder on both interfaces;

FIG. 2 an enlarged general view of a typical embodiment of the presentinvention;

FIG. 3 a description of “bow” and “smile” of laser bars;

FIG. 4 a symmetric, layered submount according to the invention;

FIG. 5 an asymmetric, layered submount;

FIG. 6 a detailed view of a specific embodiment of the invention;

FIGS. 7 a-7 b a comparison of the spectral properties over thewavelength between a laser device made by a prior art technology (FIG. 7a) and a laser device made according to the invention (FIG. 7 b);

FIGS. 8 a-8 c a more detailed comparison of the spectral propertiesbetween laser devices made by prior art technologies and laser devicesdesigned according to the invention;

FIGS. 9 a-9 b a reliability comparison between laser devices made by aprior art technology (FIG. 9 a) and laser devices designed according tothe invention (FIG. 9 b);

FIGS. 10 a-10 b the spectral behaviour and the smile shape and value ofa laser device with a planar, CTE-matched submount according to theprior art (CTE_(sub)=CTE_(bar));

FIGS. 11 a-11 b the spectral behaviour and the smile shape and value ofa laser device with submount according to the invention(CTE_(eff)<CTE_(bar));

FIGS. 12 a-12 b a comparison of smile values of two laser devices, onemade with a planar submount with CTE_(sub)<CTE_(eff) (FIG. 12 a) and oneassembled with a pre-bent submount and CTE_(sub)<CTE_(eff) (FIG. 12 b);and

FIG. 13 the initial bow of a CuMoCu submount plotted versus the finalbow of the laser device, illustrating the influence of the submountpre-bending on the final device smile.

FIGS. 1 a-1 c show three prior art embodiments of a laser diode bar on amassive copper cooler.

In the design shown in FIG. 1 a, a laser bar is directly mounted to acopper cooler. Because of the large CTE difference between the laser barand the cooler, CTE_(bar)=6.5×10⁻⁶K⁻¹ versus CTE_(cooler)=16×10⁻⁶K⁻¹,the “soft solder technology (1)” described above is used to avoidoverstressing the laser bar.

The design of in FIG. 1 b differs in that it shows a CuW submount whoseCTE matches the CTE of the laser bar, CTE_(bar)=CTE_(sub)=6.5×10⁻⁶K⁻¹.This design, above specified as technology (2a), avoids any stressbetween laser bar and submount. The stress is so-to-speak transferred tothe interface between submount and cooler where the same CTE differenceexists as in technology (1), but between other parts of the device as inFIG. 1 a. There, a soft solder is used to avoid overstressing.

FIG. 1 c shows a prior art design which uses the same materials as thedesign of FIG. 1 b, i.e. the CuW submount has about the same CTE as thecooler laser bar. However, the soft solder of FIG. 1 b between submountand cooler is replaced by a hard solder as specified in technology (2b)above. This design has the disadvantage that, depending on epitaxialdesign as well as the geometrical properties of the cooler, the stressbuilding up when the device cools down from soldering has a negativeimpact on parameters such as smile, spectral width, polarization purityand/or device reliability.

FIG. 1 d depicts a design according to the invention. Here, thesubmount's CTE_(sub) is selected such that the “combined” or “effective”CTE_(eff) of the submount (with CTE_(sub)) and the cooling element (withCTE_(cool)) differs from the laser bar's CTE_(bar) by a predeterminedamount, i.e. at least 5%, preferably about at least 10%. In absolutenumbers, the CTE difference should be at least 3-4×10⁻⁷K⁻¹.

This predetermined difference, which exerts a stress on the laser bar,distinguishes the present invention from the prior art.

There are various ways according to the invention to achieve thepredetermined difference between the CTE_(eff) of the submount/coolerassembly and the CTE_(bar) of the laser bar.

The first way is to select the submount's coefficient CTE_(sub) higherthan the laser bar's coefficient CTE_(bar), CTE_(sub)>CTE_(bar), andsmaller than or equal to the cooler's coefficient CTE_(cool),CTE_(sub)≦CTE_(cool).

The second way is to select the submount's coefficient CTE_(sub) smallerthan both the laser bar's coefficient CTE_(bar) and the cooler'scoefficient CTE_(cool), CTE_(sub)<CTE_(bar) and CTE_(sub)<CTE_(cool).

Just to repeat the basic condition defined above: all coefficients areselected such that the CTE_(eff)≠CTE_(bar).

The submount can be a solid material, e.g. an alloy or a mixture of twoor more materials. It can also be a layered structure of symmetricdesign as shown in FIG. 4 or of asymmetric design as shown in FIG. 5.For optimizing the smile and the optical properties, the submount mayhave a bow of up to 15 μm, caused by pre-bending and/or an asymmetricdesign. The smile values for a sample CuMoCu submount and an 2 mm coppercooler are shown in FIG. 10 b. Typically the laser bar is first solderedto the submount using a hard solder process, e.g. AuSn, whosesolidification temperature is typically 200-350° C. In a second solderprocess, the “bar on submount” is soldered to the copper cooler usinganother hard solder process. Alternatively, the two solder joints can beprocessed in one solder step, again using a hard solder process.Usually, the resulting thickness of the solder joints is 20 μm or lessso that they hardly affect the physical behaviour of the device.

FIG. 2 displays essentially the same device as FIG. 1 c in athree-dimensional “exploded” view. The laser bar is shown with the lightemitting areas or facets of the laser diodes. It should also be notedthat the laser bar differs from the copper cooler not only in its CTE,but also in its Young's modulus, i.e. its elasticity or E modulus, asshown in the figure. It should further be noted that the temperaturesreached during assembly of the laser device exceed the average operatingtemperature by 150-300K.

FIG. 3 explains the meaning of the term “bow” or “smile” of asemiconductor laser device as used herein, whereby the transversal orlateral bending of the device is of interest. The direction of bendingis described by either “a grumpy bow” with bow values greater than zeroor as “smiley bow” with bow values less than zero.

Constructing the submount with a CTE_(sub) that varies across thesubmount's thickness can be achieved in several ways. Such a submountwill be called a “graded-CTE submount”.

FIG. 4 shows a typical symmetric, layered design of a graded-CTEsubmount. An Mo substrate having a thickness of 300 μm and aCTE_(subB)=4.8×10⁻⁶K⁻¹ is sandwiched between two 15 μm Cu layers havinga CTE_(subA)=16×10⁻⁶K⁻¹. The two Cu layers may be plated or otherwiseapplied onto the Mo substrate. A submount of this structure and withthese dimensions has a resulting CTE_(sub) of about 5×10⁻⁶K⁻¹. FIG. 6shows a corresponding laser device in detail. The components are joinedby two different hard solders, a SnAgCu and an AuSn hard solder, usingsoldering temperatures between 200-350° C. The thickness of each solderjoint is about 20 μm or less.

Another way for constructing a graded-CTE submount is to build it ofjust two different layers, a first layer with a CTE_(subA) and a secondlayer with CTE_(subB), CTE_(subB) being different from, preferablygreater than, CTE_(subA), whereby the first layer is to be locatedadjacent the laser bar and the second layer is to be located adjacentthe cooling element. Such a submount is shown in FIG. 5, depicting atypical asymmetric, layered design of a “graded-CTE” submount. A Mosubstrate of 200 μm carries a Cu layer of 20 μm on only one side, whichis the one to be soldered to the cooler. The resulting CTE_(sub) is inthe range of 5 . . . 6×10⁻⁶ K⁻¹.

There are many other ways of constructing such a graded-CTE submount;one more being shown and described in connection with FIG. 6.

FIG. 6 is a schematic drawing of an assembled laser device according tothe invention using a graded-CTE submount. The dimensions of the laserdiode bar are 10 mm×2.4 mm×0.15 mm and its CTE_(bar)=6.5×10⁻⁶ K⁻¹.

The layered, asymmetric CuMoCu submount is 330 μm thick and consists ofa first Cu layer of 10 μm on top which faces the laser bar. The centerpart is a Mo substrate of 300 μm, and a second Cu layer of 20 μm isplaced at the bottom facing the cooler. This submount structure resultsin a CTE_(sub) of about 5×10⁻⁶ K⁻¹. The cooler is a rather rigid blockof Cu of 8 mm thickness. Both solder interfaces are made with a hardsolder process, the laser/submount interface with an AuSn hard solder,the cooler/submount interface with a SnAgCu hard solder. Please notethat all implementations of the invention show the use of hardsoldering.

The copper cooler can be either a rigid block, or it can have an innerstructure consisting of one or more water channels. The thickness rangeof the cooler is typically 1 to 10 mm.

FIGS. 7 a and 7 b compare wavelength measurements of two laser devices.A first laser device was manufactured with a prior art technology, heretechnology (2b), shown in FIG. 1 c, using a CTE-matched CuW submount andtwo hard solder processes on a 8 mm Cu cooler. FIG. 7 a shows themeasured results of this first laser device with a multi-peakcharacteristic and a rather broad spectral width. The latter makes itunsuitable for many applications.

The other laser device was made according to the invention as shown anddescribed in connection with FIG. 6. FIG. 7 b shows the result: a clean,single-peak output and a small spectral width.

FIGS. 8 a to 8 c show and compare spectral properties of varioussubmount designs in more detail than the previous FIGS. 7 a and 7 b, i.ethe influence of the value of CTE_(sub) in relation to CTE_(bar)indicated by the normalized laser beam plotted over the wavelength.

As mentioned above, a high power diode bar has a CTE_(bar) much lowerthan that of the cooler. Typically a GaAs diode bar has aCTE_(bar)=6.5×10⁻⁶K⁻¹ compared with a usual copper cooler havingCTE_(cool)=16×10⁻⁶K⁻¹. Also typically, the cooler is four to forty timesthicker than the submount.

Given these starting ranges of values, the following examples illustratehow, according to the invention, the two remaining CTEs can be selected.

FIG. 8 a shows the intensity-over-wavelength data of three laserdesigns, labeled A1, A2, and A3 below. All of them use the same laserbar.

Laser device design A1:

laser bar thickness, d_(bar)=0.15 mm, CTE_(bar)=6.5×10⁻⁶K⁻¹

submount thickness, d_(sub)=0.4 mm, CTE_(sub)=5×10⁻⁶K⁻¹

cooler thickness, d_(cool)=2 mm, CTE_(cool)=16×10⁻⁶K⁻¹.

Here, CTE_(sub) of the submount is less than the CTE_(bar) of the laserdiode bar and the CTE_(eff) is less than CTE_(bar), i.e.CTE_(eff)<CTE_(bar). The result is a tensile stress exerted onto thelaser bar. This tensile stress, which can be verified by e.g. X-raydiffraction methods, results in an optimization of the electro-opticalproperties as will be shown.

As mentioned, FIG. 8 a shows data derived from measuring three laserdevices, one of them designed according to the above specifications A1,i.e. a laser device with tensile stress. The data indicate that it isobviously a “good” laser device. This FIG. 8 a also depicts measureddata of a second “good” laser device, design A2, also made according tothe invention. Its characteristics are the following.

Laser device design A2:

laser bar thickness, d_(bar)=0.15 mm, CTE_(bar)=6.5×10⁻⁶K⁻¹

submount thickness, d_(sub)=0.4 mm, CTE_(sub)=7.5×10⁻⁶K⁻¹

cooler thickness, d_(cool)=2 mm, CTE_(cool)=16×10⁻⁶K⁻¹.

These values result in a strongly compressive stress exerted on thelaser bar, i.e. CTE_(sub)>CTE_(bar) and CTE_(eff)>>CTE_(bar).

The third curve in FIG. 8 a shows the data measured for a conventionaldesign, here a “CTE-matched” structure with the following values:

Laser device design A3:

laser bar thickness, d_(bar)=0.15 mm, CTE_(bar)=6.5×10⁻⁶K⁻¹

submount thickness, d_(sub)=0.4 mm, CTE_(sub)=6.5×10⁻⁶K⁻¹

cooler thickness, d_(cool)=2 mm, CTE_(cool)=16×10⁻⁶K⁻¹.

Looking again at FIG. 8 a, the comparison shows:

▴ Design A1: CTE_(sub)<CTE_(bar), CTE_(eff)<CTE_(bar), resultingpolarization purity>50:1;

♦ Design A2: CTE_(sub)>CTE_(bar), CTE_(eff)>>CTE_(bar), resultingpolarization purity>50:1;

▪ Design A3: CTE_(sub)=CTE_(bar), blurred and broad spectrum, resultingpolarization purity<10:1.

To summarize, design A3, which follows design rules known from the priorart by choosing CTE_(sub)=CTE_(bar), shows a blurred, broad spectrum, arather poor polarization purity and a so-to-speak unpredictable shape.It is obviously the least desirable. There may be laser applicationswhere this is unimportant, but for the plurality of applications, abetter defined laser output is required.

Much better values are measured for the two designs A1 and A2, madeaccording to the present invention, with CTE_(eff)≠CTE_(bar)—eitherCTE_(eff)<CTE_(bar) or CTE_(eff)>CTE_(bar). SelectingCTE_(sub)>CTE_(bar), CTE_(eff)>>CTE_(bar) (A1 above) leads to a muchmore precise, narrow, and better defined laser output whose spectralmaximum is close to the maximum for CTE_(sub)≈CTE_(bar) above. Thepolarization purity however of this configuration is 50:1 or more.Selecting CTE_(sub)<CTE_(bar), CTE_(eff)<CTE_(bar) (A2 above) also leadsto a spectrally well defined, precise and narrow laser output whosemaximum is shifted to longer wavelengths than for the two examplesabove. The polarization purity of this configuration is again 50:1 ormore.

Another example is illustrated in FIG. 8 b. It again shows theintensity-over-wavelength data of three laser designs, labeled B1, B2,and B3. All of them use the same laser bar.

Laser device design B1:

laser bar thickness, d_(bar)=0.15 mm, CTE_(bar)=6.5×10⁻⁶K⁻¹

submount thickness, d_(sub)=0.4 mm, CTE_(sub)=7.5×10⁻⁶K⁻¹

cooler thickness, d_(cool)=8 mm, CTE_(cool)=16×10⁻⁶K⁻¹.

Both, CTE_(sub) of the submount as well as the CTE_(cool) of the coolerare larger than the CTE_(bar) of the laser diode. Thus the CTE_(eff) isno doubt much larger than CTE_(bar), i.e. CTE_(eff)>>CTE_(bar). Theresult is a compressive stress exerted onto the laser bar. Thiscompressive stress results in the optimization of electro-opticalparameters such as a narrow spectral width and a high polarizationpurity.

The X-curve of FIG. 8 b shows the measurements of this laser device withcompressive stress, the other two with the CTE values shown. The othertwo laser devices whose curves are shown, are B2 and B3, data see belowand in the legend of FIG. 8 b. Device designs B1 to B3 have the samephysical structure and measures as device design B1.

X: CTE_(sub)=7.5×10⁻⁶K⁻¹, resulting polarization purity>50:1;

◯: CTE_(sub)=6.5×10⁻⁶K⁻¹, CTE_(sub)=CTE_(bar), CTE_(eff)>CTE_(bar)resulting polarization purity≈25:1;

Δ: CTE_(sub)=5×10⁻⁶K⁻¹, CTE_(sub)<CTE_(bar), CTE_(eff)=CTE_(bar),resulting polarization purity≈10:1.

This shows the much better performance, at least regarding thepolarization purity, of design B1 as compared to the other twoconventional designs B2 and B3 which follow the design rules of theprior art, i.e. select CTE values matching the laser bar's CTE_(bar)(B2) or minimizing the stress in the laser (B3). In other words, thoughB1 is built with design characteristics contrary to the widely acceptedgeneral design rules, it shows better results.

All in all, it should be clear from the above and the correspondingfigures that the relations CTE_(sub)≈CTE_(bar) andCTE_(eff)≈CTE_(bar)—which relations appears logical and preferable atfirst sight and allows the use of a hard solder between the submount andthe laser—are obviously not always preferable when a precise laseroutput is desired or required.

Please note that FIG. 8 a shows the results based on the relationbetween CTE_(sub) and CTE_(bar), whereas FIG. 8 b depicts measuredresults based on the relation between CTE_(eff) and CTE_(bar), CTE_(eff)being the “combined” or “overall” CTE of the submount/cooler assembly asdefined previously. Altogether are the characteristics of the threecurves similar to those of the curves in FIG. 8 a.

FIG. 8 c finally depicts the results of two laser devices with twodifferent CTE_(eff)/CTE_(bar) ratios, here not only in anotherwavelength section than shown in FIG. 8 b, but also comparing therelation CTE_(eff)≈CTE_(bar) with CTE_(eff)<CTE_(bar), i.e. where theCTE_(eff) is less than CTE_(bar), resulting in a tensile stress exertedto the laser bar. Though the spectral maxima of the two curves arepractically identical, the difference between the two curves in FIG. 8 cis very pronounced. For the data see below and in the legend of FIG. 8c.

Laser device design C1:

laser bar thickness, d_(bar)=0.15 mm, CTE_(bar)=6.5×10⁻⁶K⁻¹

submount thickness, d_(sub)=0.4 mm, CTE_(sub)=4.8×10⁻⁶K⁻¹

cooler thickness, d_(cool)=1.5 mm, CTE_(cool)=16×10⁻⁶K⁻¹.

X: CTE_(eff)<CTE_(bar), is a “tensile” design according to theinvention. Its intensity/wavelength distribution shows a precise,narrow, and well defined laser output with good polarization purity(>50:1).

Laser device design C2:

laser bar thickness, d_(bar)=0.15 mm, CTE_(bar)=6.5×10⁻⁶K⁻¹

submount thickness, d_(sub)=0.4 mm, CTE_(sub)=5.8×10⁻⁶K⁻¹

cooler thickness, d_(cool)=1.5 mm, CTE_(cool)=16×10⁻⁶K⁻¹.

◯: CTE_(eff)≈CTE_(bar), is again a conventional design. The resultingpolarization purity is ≈10:1 or less; the intensity/wavelengthdistribution is hardly useful for most applications since it is ratherwide and blurred and the polarization purity is poor (<10:1).

It is obvious from FIGS. 8 a-8 c that both relations CTE_(eff)≈CTE_(bar)or CTE_(sub)≈CTE_(bar) are the least desirable and thatCTE_(eff)≠CTE_(bar) provides significantly better results, no matterwhether CTE_(eff)>CTE_(bar) or even CTE_(eff)>>CTE_(bar) orCTE_(eff)<CTE_(bar). The measurements indicate also that the relationbetween CTE_(eff) and CTE_(bar) is the decisive relation—and not therelation between CTE_(sub) and CTE_(bar) as previously believed. Thelatter is one significant finding with regard to the present invention.Also, minimizing the assembly stress does not necessarily lead to thedesired results. Much better results are achieved by controlling and/oradjusting the assembly stress.

FIGS. 9 a and 9 b compare reliability data between two groups of laserdevices, D1 and D2 with the following characteristics.

Laser device design D1:

laser bar thickness, d_(bar)=0.15 mm, CTE_(bar)=6.4×10⁻⁶K⁻¹;

submount thickness, d_(sub)=0.4 mm, CTE_(sub)=6.4×10⁻⁶K⁻¹, graded,symmetric;

cooler thickness, d_(cool)=8 mm, CTE_(cool)=16×10⁻⁶K⁻¹.

Design D1 was built according a conventional technology (2b), shownbasically in FIG. 1 c, using a CTE-matched CuW submount,CTE_(sub)≈CTE_(bar), and two hard solder processes on a 8 mm Cu cooler.The reliability test results in FIG. 9 a show an early degradation withan operation current for 20 W output, probably because of stress-inducedemitter failures. Design D2 uses a graded symmetric CuMoCu submount,similar to the submount shown in FIG. 4.

Laser device design D2:

laser bar thickness, d_(bar)=0.15 mm, CTE_(bar)=6.4×10⁻⁶K⁻¹

submount thickness, d_(sub)=0.4 mm, CTE_(sub)=5.5×10⁻⁶K⁻¹, graded,symmetric;

cooler thickness, d_(cool)=8 mm, CTE_(cool)=16×10⁻⁶K⁻¹.

The devices of the second group were built according to the invention,as shown and described in connection with FIG. 6, i.e. with a submounthaving a CTE_(sub) of about 5.5×10⁻⁶ K⁻¹, resulting inCTE_(eff)>CTE_(bar). The reliabilty test result of this second group isshown in FIG. 9 b: a 2500 h life test with no or only little degradationof the operation current for 20 W output power. This again is a strikingexample that CTE_(sub)≠CTE_(bar) provides a far better solution withrespect to reliability than CTE_(sub)≈CTE_(bar).

FIGS. 10 a and 10 b illustrate the spectral distribution and the bow ofa laser device with a planar CuW submount with CTE_(sub)=CTE_(bar),assembled using a hard solder technology on an 2.5 mm thick copper microchannel cooler. The spectral distribution illustrated in FIG. 11 a withits breadth and double peak is far from desirable for many applications.So is the bow of this device, depicted in FIG. 11 b, which is mainlydetermined by the CTE and thickness difference between cooler andsubmount. A positive smile or bow value stands for a “smiley”, anegative bow value for a “grumpy” shape of the emitter line as describedin FIG. 3. The positive, “grumpy”, smile with 2.3 μm height is close tothe values obtained for this configuration by finite element modeling ofthe solder process.

FIGS. 11 a and 11 b show values of a laser device according to theinvention, constructed with a laser diode bar of 3.6 mm×3.6 mm×0.13 mmhard soldered to a Mo submount with CTE_(sub)=4.8×10⁻⁶K⁻¹. FIG. 12 ashows the spectral behaviour of this laser device, displaying a singlepeak and a rather narrow bandwidth. The smile of this laser device isdepicted in FIG. 12 b; it is less than 1 μm, rather close to 0.5 μm. Thelow smile value is achieved by first soldering the bar to the submountand then mounting the bar-on-submount component to the cooler, takingadvantage of the CTE difference between the Mo submount and the bar.Because of this CTE mismatch, the bar-on-submount assembly shows apre-bending in form of a “negative smile”. This cannot be achieved for abar soldered onto a planar CTE-matched submount as used in the prior artdesign described above in connection with FIG. 10. The negativebar-on-submount smile compensates for the bow caused by the secondsolder step, during which the bar-on-submount component is bent towardsthe “grumpy” direction.

The influence of pre-bending the submount (instead of thebar-on-submount) on the laser device smile is illustrated in FIGS. 12and 13. FIGS. 12 a and 12 b depict a comparison of smile or bow valuesof two complete laser devices, in both cases mounted on a rigid, about 8mm thick Cu block as cooler. FIG. 12 a shows the values for a structurewith a planar symmetric submount according to FIG. 4, soldered using ahard solder. The bow of the mounted device exceeds −2 μm, indicatingthat a pre-bending of the submount by +2-3 μm might improve the smilevalues significantly. FIG. 12 b shows the smile values for anessentially identical (except for the submount) laser device having anasymmetric submount with CTE_(sub)<CTE_(bar), e.g. according to FIG. 5and being hard soldered to the thick copper cooler: the maximum smile inthis case is less than 1 μm. For this configuration, the pre-bendsubmount has a smile of 2 μm grumpy, while the symmetric submountrelated to the smile displayed in FIG. 12 a is more or less planar. Acorrelation between final devices smile and pre-bending is shown in FIG.13 for bars assembled on CuMoCu submounts with CTE values of5-5.5×10⁻⁶K⁻¹ and a rigid Cu cooler using a hard solder technology.

Additional advantages and modifications will readily occur to personsskilled in the art and the invention is therefore not limited to thespecific embodiments, details, and steps shown and describedhereinbefore. Modifications may be made without departing from thespirit and scope of the general inventive concepts as defined in theappended claims.

The invention claimed is:
 1. A method for making a high power lasersource of more than one W, said laser source including a bar of laserdiodes having a light-emitting active semiconductor region, a coolingelement and a submount between said laser bar and said cooling element,said laser bar having a first coefficient of thermal expansion(CTE_(bar)), said submount having a second coefficient of thermalexpansion (CTE_(sub)), said cooling element having a third coefficientof thermal expansion (CTE_(cool)), and a submount/cooler assemblyconsisting of said submount and said cooling element, saidsubmount/cooler assembly having an effective fourth coefficient ofexpansion (CTE_(eff)), wherein selecting said fourth coefficient(CTE_(eff)) different by a predetermined amount from said firstcoefficient (CTE_(bar)), CTE_(eff)≠CTE_(bar), hard soldering said bar oflaser diodes to said submount and hard soldering said submount to saidcooling element, thereby stressing said laser bar's active region toeffect a deformation of its semiconductor crystal lattice.
 2. The methodaccording to claim 1, wherein the predetermined amount of differencebetween the first coefficient (CTE_(bar)) and the fourth coefficient(CTE_(eff)) is selected to be at least 5%.
 3. The method according toclaim 1, wherein the predetermined amount of difference between thefirst coefficient (CTE_(bar)) and the fourth coefficient (CTE_(eff)) isselected to effect a deformation of the crystal lattice of at leastabout 0.01%.
 4. The method according to claim 1, wherein said secondcoefficient (CTE_(sub)) is selected higher than said first coefficient(CTE_(bar)) and smaller than or equal to said third coefficient(CTE_(cool)): CTE_(sub)>CTE_(bar) and CTE_(sub)≦CTE_(cool).
 5. Themethod according to claim 1, wherein said second coefficient (CTE_(sub))is selected smaller than both said first coefficient (CTE_(bar)) andsaid third coefficient (CTE_(cool)): CTE_(sub)<CTE_(bar) andCTE_(sub)<CTE_(cool).
 6. The method according to claim 1, wherein thetwo soldering steps are executed simultaneously.
 7. The method accordingto claim 1, wherein the soldering steps are executed at temperatures ofabout 200-350° C.
 8. The method according to claim 1, wherein thesubmount or part of said submount or the assembly consisting of saidsubmount and said cooling element is pre-bent.